The invention relates to tower cranes, and more particularly to improved lifting and supporting apparatus for such a crane, and an associated method for lifting and supporting the crane.
Tower cranes have been used increasingly in recent years, particularly for the construction of reinforced concrete buildings. Some are free-standing, but the type of tower crane to which this invention relates is positioned within the building under construction, supported on building structure, usually passing through aligned floor slab openings created for this purpose. The tower of the tower crane is usually supported from one floor, extending upward through aligned openings in all completed upper floors.
As construction of the building progresses, the floors approach the boom of the crane, necessitating the raising of the boom and tower to a higher elevation.
For lifting the tower, one system previously in common use employed a hydraulic cylinder connected to a specially equipped lower mast section, with an additional cross piece or traverse connected to the cylinder and engaged in a pair of opposed ladder-like columns extending from the lower floors to the top of the building alongside the tower, within the shaft of floor openings. Dogs of this lifting structure would engage on rungs or slots of these ladder devices, moving up the ladders and engaging new slots as the lifting cylinder was actuated. Since the ladder devices extended through the entire height of the building, they served as supporting means for the working crane, as well as being involved in the lifting operation when raising of the crane was required.
In another lifting system which has been widely used, jacks were positioned on a floor of the building structure, with smooth lifting rods depending downwardly from the jacks to a connecting device engagable with a specially equipped mast section. A device associated with the jacks and having teeth would grasp each rod for the lifting stroke. The specially equipped mast section, normally located at the bottom of the tower crane, had holes for receiving the connecting device, which extended laterally through the mast. The tower crane was supported entirely by the rods and connecting device while being lifted by a series of jacking strokes.
For supporting the crane after it was lifted to the new elevation, this latter system utilized I-beams inserted horizontally through a mast section. The jacks lowered the crane a short distance, to rest the I-beam on a building floor.
Both these prior systems and methods for lifting and supporting a tower crane were effective, but required specially equipped mast sections and additional heavy equipment and were usually time consuming to make ready for a lifting operation. Also, the support systems for the working crane were inadequate in that they depended upon the position of the specially fitted mast section which was designed for the support function. It is always highly desirable to have a strong point of the tower, where diagonals meet horizontal and vertical members, at the level of the highest floor through which the tower extends, to accept side loading forces to hold the crane from tipping and to accept twisting forces induced by the boom. If the highest floor does not coincide with a strong point, but rather lies between two of them, the side loading and twisting forces on the tower can bend and buckle the unsupported stretches of vertical corner members in the tower. With some support systems, such positioning has been unavoidable because the spacing between floors is usually not the same as or an integral multiple of the spacing between the strong points of the mast sections. Therefore, it has been necessary to strengthen the vertical mast members or use some other arrangement to distribute the side loading over a larger area or to the strong points.
Another shortcoming of most prior tower lifting systems were that they were required to be left with one particular crane during the entire construction operation. It was impossible to utilize them for multiple cranes at the same construction site.